Was Einstein Wrong About Space Travel?

March 22, 2006: Consider a pair of brothers, identical
twins. One gets a job as an astronaut and rockets into deep
space. The other stays on Earth. When the traveling twin returns
home, he discovers he's younger than his brother.

This
is Einstein's Twin Paradox, and although it sounds strange,
it is absolutely true. The theory of relativity tells us that
the faster you travel through space, the slower you travel
through time. Rocketing to Alpha Centauri—warp 9, please—is
a good way to stay young.

Or
is it?

Some
researchers are beginning to believe that space travel could
have the opposite effect. It could make you prematurely old.

Above:
Albert Einstein's theory of Special Relativity says that time
slows down for fast-moving space travelers, effectively keeping
them young. Space radiation acting on telomeres could reverse
the effect. [More]

"The
problem with Einstein's paradox is that it doesn't fold in
biology—specifically, space radiation and the biology of aging,"
says Frank Cucinotta, NASA's chief scientist for radiation
studies at the Johnson Space Center.

While
the astronaut twin is hurtling through space, Cucinotta explains,
his chromosomes are exposed to penetrating cosmic rays. This
can damage his telomeres—little molecular "caps"
on the ends of his DNA. Here on Earth, the loss of telomeres
has been linked to aging.

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So
far, the risk hasn't been a major concern: The effect on shuttle
and space station astronauts, if any, would be very small.
These astronauts orbit inside of Earth's protective magnetic
field, which deflects most cosmic rays.

But
by 2018, NASA plans to send humans outside of that protective
bubble to return to the moon and eventually travel to Mars.
Astronauts on those missions could be exposed to cosmic rays
for weeks or months at a time. Naturally, NASA is keen to
find out whether or not the danger of "radiation aging"
really exists, and if so, how to handle it.

Science
is only now beginning to look at the question. "The reality
is, we have very little information about [the link between]
radiation and telomere loss," says Jerry Shay, a cell
biologist at the University of Texas Southwestern Medical
Center at Dallas. With support from NASA, Shay and others
are studying the problem. What they learn about aging could
benefit everyone, on Earth and in space.

A
Lit Fuse

Like
the fuse of a time bomb, telomeres are long strands of repeating
DNA that shorten each time a cell divides. When the telomeres
become too short, the cell's time is up: It can no longer
divide, a state of affairs known as "replicative senescence."

Without
this built-in fuse, human cells would be able to continue
growing and dividing indefinitely. In fact, scientists believe
that cells evolved telomeres as a way of preventing the out-of-control
cell growth of cancerous tumors. Because of telomeres, most
human cells can only divide 50 to 100 times before the time
bomb goes off.

One
current theory of aging holds that, as the cells of a person's
body start to hit this telomere-imposed limit, the lack of
fresh, new cells causes the typical signs of aging: wrinkled
skin, failing organs, weaker immune system, etc.

Whether
or not telomere loss actually causes aging remains a matter
of debate, Shay notes. The fact that shortened telomeres go
hand in hand with aging is well documented. People with shorter
telomeres, for example, are known to not live as long on average
as people with longer telomeres. But mere correlation doesn't
prove whether telomeres are in fact the cause.

"It's
hard to prove cause and effect in these things. But I think
there's a sufficient number of these correlative studies from
a variety of different investigators that one has to start
believing that short telomeres are a marker of aging,"
Shay says.

To
prove this, they exposed laboratory dishes containing a kind
of human blood cell called lymphocytes to beams of both iron
nuclei and gamma rays. Until recently, such a thorough analysis
of telomere damage would have been prohibitively time consuming.
But a new cell-staining technique called RxFISH (Rainbow cross-species
Fluorescence In Situ Hybridization) allowed Cucinotta and
his colleagues to look at many telomeres simultaneously.

"We
had this surprising result that iron particles are much more
damaging to telomeres than gamma rays," Cucinotta says.
He suggests that this difference might be due to the wider
path of damage caused by iron nuclei. Telomere strands wrap
into elongated loops, like little knots on the ends of chromosomes.
Gamma rays can only strike one side of these loops or the
other, but iron nuclei can affect both sides at the same time,
inflicting lasting damage on the telomere—possibly causing
its complete deletion. This explanation is still speculative,
however.

The
task now is to quantify the risk telomere damage might pose
to astronauts, so that mission managers and the astronauts
themselves can make informed decisions about the risks they
face. In all likelihood, the effects will be modest, Shay
says.

"We're
talking about subtle things. These people are probably not
going to wind up in wheelchairs or something like that from
being in space," Shay says.

For
example, astronauts who have had the greatest exposure to
space radiation, such as the Apollo astronauts who traveled
to the Moon, tend to get cataracts about 7 years earlier than
other astronauts, on average. Cataracts are a common symptom
of aging.

Of
greater concern is possible aging of the brain and spinal
cord. Experiments with rats have shown that brain tissue is
vulnerable to "aging" by iron-nuclei radiation--this
according to research by Jim Joseph of Tufts University and
Bernie Rabin at the University of Maryland. (See references
below.)

"It
is looking more and more likely that this could be a problem
for long-term space travel," Cucinotta says.

However,
if scientists can tease apart the exact ways that iron-particle
radiation affects telomeres, they may be able find a way to
avoid or correct it. The solution could be as simple as a
pill containing DNA-repair molecules. "There are many
ways that we can intervene," Shay says.

One
way or another, NASA plans to keep their astronauts feeling
young.

Editor's
note: This story should not be construed to mean that Einstein's
theory of Special Relativity is wrong. It is correct. The
Twin Paradox was concocted in Einstein's day to illustrate
time dilation only. It was never intended to treat all aspects
of space travel. The newly discovered effect of space radiation
on telomeres is the "paradox on the paradox," says
Frank Cucinotta.